Steam turbine power plants



United States Patent 3,238,729 STEAM TURBINE POWER PLANTS Adolf Frankel,Altrincham, Cheshire, and Peter Harding Brazier, Sale, Cheshire,England, assignors to Associated Electrical Industries Limited, London,England, a British company Filed July 15, 1963, Ser. No. 295,035 Claimspriority, application Great Britain, July 23, 1962, 28,277/62 Claims.(CI. 60-67) This invention relates to improvements in steam turbinepower plants, and more particularly to steam turbine power plants inwhich feed water is heated by steam bled from the steam turbine.

It is usual in large power plants to use a number of feed water heatersheated respectively by steam flows from dilferent stages in the turbineand acting in turn upon the feed water, the hotter steam being used toheat the hotter feed water, since this increases the thermal efficiencyof the steam turbine power plant.

Such a feed Water heater may contain desuperheating, condensing anddrain cooling sections. In a condensing section, steam tapped from asuitable stage in the turbine is condensed, so heating the feed water upto a temperature below the saturation temperature of the steam by thetemperature difference required to effect the required transfer of heatfrom the steam to the feed Water. The saturation temperature in theheater depends upon the steam pressure in the heater, and this is equalto the steam pressure at the turbine stage from which the steam istapped less the pressure drop between the turbine stage and thecondensing section of the feed water heater. Some pressure drop will benecessary to overcome the resistance to flow of steam through theassociated steam pipes or mains. If the tapped steam has a high degreeof superheat, the feed water may be heated further after the condensingsection by the transfer of the sensible heat, providing the superheat,to the feed water in a desuperheating section, the desuperheated steampassing on to the condensing section of the feed water heater. Thepressure drop of the tapped steam in the desuperheater has the effect offurther depressing the saturation temperature of the heating steam inthe condensing section, so that the temperature of the feed waterleaving the condensing section is lowered by the use of a desuperheatingsection.

An object of the present invention is the provision of a steam turbinepower plant having an improved feed water heating system.

According to the present invention, in a steam'turbine power plantincluding a steam turbine and feed Water heating means arranged to heatfeed Water passing to a steam generating unit supplying the steamturbine, a feed water heater of the feed water heating means comprises afirst condensing section arranged to heat feed water and a secondcondensing section arranged further to heat water discharged from thefirst section, the two sections being supplied with heating steam byparallel connections from the same bleed point on the steam turbine orfrom equivalent bleed points on cylinders operating in parallel, and thepressure of the steam entering the second section being greater than thepressure of the steam entering the first section, whereby the saturationtemperature of the steam in the second section is greater than thesaturation temperature of the steam in the first section.

Since both sections of the feed heater obtain steam from the same bleedpoint on the steam turbine, an additional pressure drop is available inthe steam supply system to the first section of the feed heater, and inaccordance with the invention this additional pressure drop is availablefor "ice an economically useful application, for instance, forincreasing the velocity and reducing the size of steam supply pipes, forincreasing the pressure drop across a desuperheating section, andthereby decreasing its size, for passing the steam through an ejectorentraining additional steam quantities from a lower pressure level, orfor any other economically or thermodynamically practicable application.

Preferably the second section is arranged to impart to the feed wateronly some 15 to 25% of the total temperature rise imparted to the feedwater in both sections.

By way of example, one of the two connections may convey superheatedsteam first to a third desuperheating section arranged to receive heatedfeed Water from the second section, the desuperheated steam flowing onto the first section, while the second connection conveys superheatedsteam with little pressure drop to the second section.

The invention will now be described, by way of example with reference tothe accompanying drawings, in which:

FIGURE 1 is a schematic diagram of a steam turbine power plant to whichthe invention is applied; and

FIGURE 2 is a sectional side elevation of a tubulous desuperheating feedWater heater.

The steam turbine power plant includes a steam generating andsuperheating unit 1, a steam turbine having a high pressure cylinder 3Hand two parallel connected low pressure cylinders 3L, and a steamcondenser 5.

The steam generating and superheating unit 1 is shown diagrammaticallyas consisting of upper and lower drums 7 and 9 respectively, connectedtogether by a bank of tubes 11, and a superheater 13 arranged to receivesaturated steam from the upper drum 7 and to pass superheated steamthrough a steam main 15 to the inlet of the turbine high pressure stage3H. The exhaust of the stage 3H is connected by a steam main 17 to thecommon inlet of the two lower pressure stages 3L, and the exhausts ofthe two stages are connected separately to the condenser 5. Coolingwater passes through the condenser in a bank of cooling tubes indicateddiagrammatically at 19. The detailed constructions of all of theforegoing items are well known in the power plant art.

Condensate collecting in the condenser 5 is removed by a condensate pump21 and from the pump outlet pipe 22 passes through feed water heaters23, 25, 27, 29' and 31 in turn. All these feed water heaters are of theindirectheat-transfer type, in which the feed Water passes through abank of tubes contained in a cylindrical shell, and heating steam iscaused to flow through the shell over the outsides of the tubes. In thecase of heaters 27 and 29, they are combined in the form of a singlebank of tubes passing through a single shell formed with a partition 28which serves to separate the steam flow in heater 27 from that in heater29. Heaters 23 and 25 are similarly combined.

The high pressure turbine stage 3H is provided with a higher pressurebleed 35 which bifurcates into a low velocity steam conduit 35S and ahigh velocity steam conduit 35F. The low pressure stage 3L is providedwith a lower pressure bleed 37 which bifurcates into a low velocitysteam conduit 37S and a high velocity steam conduit 37F. Conduits 35F,35S, 37S and 37F are connected to the steam inlets of the feed waterheaters 31, 29, 25 and 23 respectively. Feed water heater 31, adesuperheater, is provided with a steam outlet which is connected to thesteam inlet of heater 27. The condensate outlets from the heaters 29 and25 are connected respectively by condensate pipes 41 and 42 to the waterspaces of heaters 27 and 23 respectively, and heater 23 is arranged todischarge condensate into the condenser 5.

During operation of the power plant, condensate from the condenser 5 isfed by the pump 21 through the feed water heaters to the water space ofthe drum 7. Steam separated in the drum 7 passes through the superheater13 to the turbine high pressure stage 3H, in which most of the steam isexpanded and passes on through steam main 17 to the two low pressurestages 3L. Steam exhausted from these stages SL is condensed in thecondenser 5.

Suitable amounts of steam are bled from the turbine stages 3H and 3Lthrough the bleeds 35 and 37, and pass through conduits 35F, 355, 37-5and 37F to the feed water heaters 31, 29, 25 and 23. The steam enteringthe heaters 29, 27, 25 and 23 is condensed, but the superheated steamentering heater 31 is for the most part only fully or partlydesuperheated and then passes on to the heater 27, in which it iscondensed.

The dimensions of the conduits 37F and 37S and of the associated feedwater heaters 23 and 25 are such that the mass flow rate of steamthrough the conduit 37F is considerably larger than that in conduit 37Sand amounts to some 75 to 85% of the steam bled through the bleed 37. Atthe same time, the cross-sectional area of conduit 378 is so selectedthat the velocity of the steam flowing through it is much smaller, forexample one third, than the velocity of the steam flowing in the conduit37F.

Such an arrangement results in the use of a total crosssectional area ofconduit which is of the order of 50 to 80% of the cross-sectional areaof conduit which would be used in a conventional installation, in whichthe conduit conveying the steam from bleed 37 would need to be of suchcross-sectional area that the pressure drop along the conduit was notexcessive.

The dimensions of the conduits 35F and 355 are similarly chosen so thatthe steam mass flow rate in conduit 35F amounts to some 75 to 85% of thesteam bled through the bleed 35.

The pressure drop produced in the conduit 37F is much larger than thatproduced in the conduit 375, while the pressure drop produced in theconduit 35F and the desuperheater 31 is much larger than that producedin the conduit 355. In the case of each bleed 35 and 37, the steam bledfrom the steam turbine to a given feed water heater as shown is splitinto two flows, one of which carries considerably more than half of thetotal steam flow. This larger steam flow is subjected to a pressure dropand is then supplied to the feed heater means. Because of the pressuredrop mentioned above, the saturation pressure and the saturationtemperature of that portion of the steam will be lower than thoseobtainable if the steam was supplied to the feed water heater means in amore direct way with a minimum of pressure drop. Therefore, the finaltemperature to which it will be possible to heat the feed water in theportion of the feed water heater means to which this steam is suppliedwill be correspondingly reduced. In accordance with the invention, thesecond and smaller part of the steam flow is supplied to a secondcondensing part of the feed water heater means, arranged in series withthe first condensing part, in such a way as to involve the minimumpracticable pressure drop. As a result the feed water is heated in thesecond part of the feed water heater means to the highest possibletemperature obtainable with the given saturation temperature of thesteam, so that the provided pressure drop to the second section of thefeed water heater is no greater than the pressure drop which isvnormally economically acceptable between the turbine and the feed waterheater of a conventional arrangement, the invention does not involve anyadditional thermodynamic loss compared with the conventionalarrangement. If the temperature rise in the second section of the feedwater heater is arranged, by suitable proportioning of the pressuredrops to each section, to be a small proportion of the total temperaturerise in both sections, say 15 to 25%, then the steam flow to thissection is small and a steam velocity and hence the pressure dropbetween the turbine and the heater which are less than those in aconventional arrangement can be economically justified. This wouldresult in a thermodynamic improvement compared with the conventionalarrangement.

As shown above the pressure drop involved by the processes to which thesteam to the first section of the feed water heater is subjected, doesnot involve any additional thermodynamic loss, and hence may be used fora number of purposes with advantage, for example in the case describedabove with rerference to heaters 23 and 25 where only 15 to 25% of theheating is carried out in the second section of the feed water heater,to of the heating must be carried out in the first section of the feedwater heater requiring 75 to 85% of the total steam to the feed waterheater. Since the pressure drop required to cause the steam to flow fromthe turbine to the first section of the feed water heater does notinvolve any additional thermodynamic loss, it may be transferred as amuch higher velocity than would be justified economically with theconventional arrangement, so permitting a considerable reduction in thesize of pipe through which this steam fiows.

Alternatively, if the tapped steam to the feed water heater issuperheated, then the process to which the steam to the first section ofthe feed water heater is involved, can include a feed water heater wherethe steam is desuperheated, as described above in connection withheaters 31, 29 and 27, before passing to the first section of the feedwater heater where it is condensed. Since the pressure drop to the firstsection of the feed water heater does not involve any additionalthermodynamic loss, the pressure drop required to pass the steam throughthe desuperheater may be much greater than would be economicallyjustified in a conventional feed water heating arrangement, and as aresult the heat transfer coefficients will be greater and the size andcost less than with the conventional arrangement. If the feed Water tothe desuperheater is taken from after the second section of the feedwater heater, the inlet feed water temperature to the desuperheater,which is the outlet feed water temperature from the second section ofthe feed water heater, is unaffected by the pressure drop of the steampassing through the desuperheater. The invention thus overcomes thecompromise which has to be made in the conventional feed water heatingarrangement when the steam velocities in the desuperheater are very lowso as to prevent excessive depression of the outlet feed watertemperature of the condensing section due to the pressure drop throughthe desuperheater, and as a result requires a very large desuperheater.The feed water temperature out of the second section of the feed waterheater is not depressed at all by the desuperheater, so that thearrangement should show a thermodynamic gain compared with theconventional arrangement, as well as a cheaper desuperheater due to thereasons given above.

The conduit 35S can also include an ejector 38 through the nozzle ofwhich the steam flowing in the conduit fiows and the suction chamber ofwhich is arranged to entrain additional steam from the steam bleed 37which operates at a steam pressure lower than that of the steam bleed35.

It would also, of course, be quite practicable, and in accordance withthe teaching of the invention, to run only one pipe line from thetapping point on the turbine to the vicinity of the heaters 27, 29, 31,with the pipe branching out to supply heater 29 and desuperheater 31 asnear to these heaters as convenient. Because of the pressure drop acrossdesuperheater 31, even if the pressure drop through the short branchlines to heaters 29 and 31 was equal (in any case it is likely to benegligible) the steam supply pressure to heater 27 will be lower thanthe steam supply pressure to heater 29, which is the essence of theinvention.

The invention is applicable to feed water heaters using heat transfersurfaces, which are usually of the type which feed water flows insideheat transfer tubes with the heating steam condensing on the outsides ofthe tubes, and to direct contact heaters in which the feed water flows,or is sprayed, in a chamber to which the heating steam is supplied. Inthe case of indirect heat transfer feed water heater means, the twoparts of the heater which are arranged in series either can be arrangedin separate vessels or a suitable bafile can be provided inside onecommon vessel, separating the two parts which operate at differentpressure and temperature levels. As the diiference in pressure andtemperature involved is not very large, even appreciable leakage ofheating steam from the high temperature final part of the feed waterheater means to the main part of the feed water heater means isthermodynamically insignificant. There is little additionalthermodynamic loss in such a leakage. Under the circumstances a simplebathe in one common casing is mechanically practical.

FIGURE 2 illustrates a single feed water heater which includes sectionscorresponding to feed water heaters 31, 29 and 27 in FIGURE 1. A groupof nested U-tubes 51 are mounted on a tube plate 53 so that their inletends lie opposite an inlet chamber 55 and their outlet ends lie oppositean outlet chamber 57 provided in a body 59 to which the tube plate issecured. An inlet pipe 61 communicates with the chamber 55 and would becon nected to the feed water oulet from heater 25, while an outlet pipe63 communicates with the chamber 57 and would be connected to the drum7. The tubes 51 are enclosed by a shell 65, and bafile plates 71, 72, 73and 74 fitted to the tubes 51 before assembly and battle plate 75secured to the tube plate 53 define, together with the shell, threeseparate chambers. 74 define a sinuous steam flow passage leading from asteam inlet 77 and extending about the outlet ends of the tubes 51through a passage in the bafile plate 75 into the main part of theshell. Bafiles 71 and 72 define a closed chamber having a steam inlet 79and a condensate escape hole equivalent to and given the same referenceas condensate pipe 41. The remainder of the space inside the shell formsthe third chamber and is provided with a condensate drain equivalent tothe condensate pipe 42.

When the invention is applied to direct contact feed water heater means,the two parts of the heater can be arranged in separate vessels, or theycan both be accommodated in one common vessel with an internal partitionwall separating the two parts. For the reasons referred to above, thepartition wall separating the two parts need not be very strongstructurally. Also, if the final part of the feed Water heater means isarranged below its first part, arrangements can easily be made for themixture of the feed water and the condensed heating steam from the firstpart to flow by gravity, through suitable loop pipes, into the secondpart. As a result of such an arrangement, the total volume of the directcontact feed water heater will be smaller than the volume of theconventionally arranged feed water heater, because a smaller volume forthe required heat turnover can be provided in the first part of the feedwater heater than would otherwise be considered prudent. The only effectof such a reduction of volume will be a modest increase in terminaltemperature dilference between the feed water leaving this portion ofthe feed water heater and the saturation temperature of the steam atthis point. As explained above this does not involve a thermodynamicloss as long as the second portion of the heater means, handling a muchsmaller steam flow, is suitably dimensioned and designed.

The volume to be provided in the first part of the direct contact feedwater heater means, arranged in the way described above, can be furtherreduced by arrang- Thus baflles 72, 73 and ing in the partition wallbetween the two halves suitable perforations and/or stubs projectinginto the bottom of the first part. Steam from this final part of theheater means will then be injected into the first part of the heatermeans, and in the process, if the stub pipes are suitably arranged, itwill entrain large quantities of the feed water lying at the bottom ofthe first part of the heater means, and project that water into the openspace above the general water level. As explained above, such a leak ofsteam from one part to the other does not involve a direct thermodynamicloss. The effect ofthe disturbance created by the process describedabove will be to expose a much larger surface of the water to theheating steam supplied to the first part of the heater means, i.e. toproduce the same elfect as that produced by sprays and drip trays indirect contact heat exchanges. Due to that effect the heat exchangerequired for condensation of the steam in the first part of the feedwater heater can be carried out in a very much smaller volume than wouldotherwise be necessary. As the bulk of the heat exchange in the feedwater heater means is in the first part, this can result in aconsiderable reduction in the volume of the heater means.

In some instances it may prove advantageous to split the total flow ofsteam to the heater means into three or even more sections, each ofwhich is subject to a diiferent degree of pressure drop by flowing tothe heater means through pipes at different velocities. They would besupplied to successive parts of the heater means, the steam flowsubmitted to the largest pressure drop passing to the feed water heaterat the entry to the heating means and the steam flow subjected to theminimum pressure drop passing to the feed water heater at the outletfrom the heating means.

For example, in machines using three low pressure cylinders or stages,it may be particularly advantageous to split each of the feed waterheaters into three sections, so that steam bleeds from the threeequivalent bleed points on the parallel connected low pressure stagesflow respectively to the three sections.

What we claim is:

1. A steam turbine power plant including:

(a) a steam turbine;

(b) feed water heating means arranged to heat feed water passing to asteam generating unit supplying the steam turbine;

(c) a first condensing section of the feed water heating means arrangedto heat the feed water;

((1) a second condensing section of the feed water heating meansarranged further to heat water discharged from the first section;

(e) a bleed point on the steam turbine; and

(f) first and second bleed conduits connected to said bleed point tofurnish bleed steam to the first and second condensing sectionsrespectively, means for proportioning the pressure in said first andsecond bleed conduits so that the pressure of the steam entering thesecond condensing section is greater than that entering the firstcondensing section where.

by the saturation temperature of the steam in the second condensingsection is greater than the saturation temperature of the steam in thefirst condensing section.

2. A power plant according to claim 1, in which the first bleed conduitwhich furnishes bleed steam to the first condensing section is arrangedto operate with a higher steam velocity through it than is the secondbleed conduit which furnishes bleed steam to the second condensingsection.

3. A power plant according to claim 1, in which the first bleed conduitwhich furnishes bleed steam to the first condensing sections includes adesuperheating feed water heater of the feed water heating meansarranged further to heat feed water from the second condensing section.

4. A power plant according to claim 1, in which the first bleed conduitwhich furnishes bleed steam to the first condensing section includes anejector through the nozzle of which the steam flowing in the first bleedconduit flows and the suction chamber of which is arranged to entrainadditional steam from a steam bleed operating at a steam pressure lowerthat that of the steam supplied to the first and second bleed conduits.

5. A power plant according to claim 1, in which the second section isarranged to impart to the feed water only some 15 to 25 percent of thetotal temperature rise imparted to the feed water in both sections.

6. A power plant according to claim 1, in which a third bleed conduit isarranged to supply heating steam to a third condensing section from thesame bleed point on the turbine, the pressure of the steam entering thethird section being intermediate the pressure of the steam entering thefirst and second sections and the third section being arranged to heatwater flowing from the first section to the third section.

7. A power plant according to claim 1, in which the feed water sectionsare indirect heat exchangers including tubes through the walls of whichheat is transferred from the steam to the feed water.

8. A feed water heater suitable for use in a power plant according toclaim 7, and including a bank of 'U-shaped tubes arranged inside a shelland connected at their ends respectively to inlet and outlet chambersfor the feed water, the space inside the shell outside the tubes beingdivided into a first compartment through which the parts of the tubesadjacent the outlet chamber extend, a second compartment through whichthe parts of the tubes adjacent the inlet chamber extend, and a thirdcompartment through which the intermediate parts of the tubes extend, asteam inlet for a first flow of heating steam into the firstcompartment, a steam inlet for a second and separate flow of heatingsteam into the third compartment, and passage means by which cooledfluid from the first compartment and condensate from the thirdcompartment can flow into the second compartment, and a drain forcondensate from the second compartment.

9. A steam turbine power plant including:

(a) a steam turbine;

(b) feed water heating means arranged to heat feed water passing to asteam generating unit supplying the steam turbine;

(c) a first condensing section of the feed water heating means arrangedto heat the feed water;

(d) a second condensing section of the feed water heating means arrangedfurther to heat water discharged from the first section;

(e) equivalent bleed points on cylinders of the turbine which arearranged to operate in parallel; and

(f) first and second bleed conduits connected to said bleed point tofurnish bleed steam to the first and second condensing sections andrespectively, means for proportioning the pressure in said first andsecond bleed conduits so that the pressure of the steam entering thesecond condensing section is greater than that entering the firstcondensing section whereby the saturation temperature of the steam inthe second condensing section is greater than the saturation temperatureof the steam in the first condensing section.

10. A power plant according to claim 9, in which the first bleed conduitwhich furnishes bleed steam to the first condensing section is arrangedto operate with a higher steam velocity through it than is the secondbleed conduit which furnishes bleed steam to the second condensingsection.

11. A power plant according to claim 9, in which the first bleed conduitwhich furnishes bleed steam to the first condensing sections includes adesuperheating feed water heater of the feed water heating meansarranged further to heat feed water from the second condensing section.

12. A power plant according to claim 9, in which the first bleed conduitwhich furnishes bleed steam to the first condensing section includes anejector through the nozzle of which the steam flowing in the first bleedconduit flows and the suction chamber of which is arranged to entrainadditional steam from a steam bleed operating at a steam pressure lowerthan that of the steam supplied to the first and second bleed conduits.

13. A power plant according to claim 9, in which the second section isarranged to impart to the feed water only some 15 to 25 percent of thetotal temperature rise imparted to the feed water in both sections.

14. A power plant according to claim 9, in which a third bleed conduitis arranged to supply heating steam to a third condensing section froman equivalent bleed point on a cylinder operating in parallel to thesaid cylinders, the pressure of the steam entering the third sectionbeing intermediate the pressures of the steam entering the first andsecond sections and the third section being arranged to heat waterflowing from the first section to the third section.

15. A power plant according to claim 9, in which the feed water sectionsare indirect heat exchangers including tubes through the walls of whichheat is transferred from the steam to the feed water.

16. A feed water heater suitable for use in a power plant according toclaim 15, including a bank of U-shaped tubes arranged inside a shell andconnected at their ends respectively to inlet and outlet chambers forthe feed water, the space inside the shell outside the tubes beingdivided into a first compartment through which the parts of the tubesadjacent the outlet chamber extend, a second compartment through whichthe parts of the tubes adjacent the inlet chamber extend, and a thirdcompartment through which the intermediate parts of the tubes extend, asteam inlet for a first flow of heating steam into the firstcompartment, a steam inlet for a second and separate flow of heatingsteam into the third compartment, and passage means by which cooledfluid from the first compartment and condensate from the thirdcompartment can flow into the second compartment, and a drain forcondensate from the second compartment.

17. Method of operating a power plant including a steam turbine and feedwater heating means arranged to heat feed water passing to a steamgenerating unit supplying the steam turbine, comprising bleeding steamfrom the turbine through first and second bleed conduits connected tothe same bleed point on the turbine, supplying steam to a firstcondensing section of the feed water heating means from the first bleedconduit and supplying steam to a second condensing section of the feedwater heating means from the second bleed conduit proportioning thepressure of the steam in the first and second conduits so that thepressure of the steam entering the second section is greater than thepressure of the steam entering the first section, and heating feed waterin the first condensing section and then heating this water further inthe second condensing section.

18. The method according to claim 17, in which the steam flowing throughthe first bleed conduit towards the first section is caused to flowthrough a desuperheating feed water heater of the feed water heatingmeans and the feed water leaving the second condensing section isfurther heated in the desuperheating feed water heater.

19. Method of operating a power plant including a steam turbine and feedwater heating means arranged to heat feed water passing to a steamgenerating unit supplying the steam turbine, comprising bleeding steamfrom the turbine through first and second bleed conduits connected toequivalent bleed points on cylinders operating in parallel, supplyingsteam to a first condensing section of the feed water heating means fromthe first bleed conduit and supplying steam to a second condensingsection of the feed water heating means from the second bleed conduit,

proportioning the pressure of the steam in the first and second conduitsso that the pressure of the steam entering the second section is greaterthan the pressure of the steam entering the first section, and heatingfeed water in the first condensing section and then heating this waterfurther in the second condensing section.

20. The method according to claim 19, in which the steam flowing throughthe first bleed conduit towards the first section is caused to flowthrough a desuperheating feed water heater of the feed water heatingmeans and the feed water leaving the second condensing section isfurther heated in the desuperheating feed water heater.

References Cited by the Examiner UNITED STATES PATENTS 1,750,035 3/1930Brown 60- 67 X 5 1,781,368 11/1930 Davidson 60-107X 1,846,047 2/1932Brown 6067 OTHER REFERENCES German printed application No. 1,007,779,May 1957.

10 SAMUEL LEVINE, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

1. A STEM TURBINE POWER PLATE INCLUDING: (A) A STEAM TURBINE; (B) FEEDWATER HEATING MEANS ARRANGED TO HEAT FEED WATER PASSING TO A STEAMGENERATING UNIT SUPPLYING THE STEAM TURBINE; (C) A FIRST CONDENSINGSECTION OF THE FEED WATER HEATING MEANS ARRANGED TO HEAT THE FEED WATER:(D) A SECOND CONDENSING SECTION OF THE FEED WATER HEATING MEANS ARRANGEDFURTHER TO HEAT WATER DISCHARGED FROM THE FIRST SECTION; (E) A BLEEDPOINT ON THE STEAM TO THE FIRST AND (F) FIRST AND SECOND BLEED CONDUITSCONNECTED TO SAID BLEED POINT TO FURNISH BLEED STEAM TO THE FIRST ANDSECOND CONDENSING SECTIONS RESPECTIVELY, MEANS FOR PROPORTIONING THEPRESSURE IN SAID FIRST AND SECOND BLEED CONDUIT SO THAT THE PRESSURE OFTHE STEAM ENTERING THE SECOND CONDENSING SECTION IS GREATER THAN THEENTERING THE FIRST CONDENSING SECTION IS GREATER BY THE SATURATIONTEMPERATURE OF THE STEAM IN THE SECOND CONDENSING SECTION IS GREATERTHAN THE SATURATION TEMPERATURE OF THE STEAM IN THE FIRST CONDENSINGSECTION.